The tropical intraseasonal (30-50 day) oscillation manifests itself in the
Australian summer monsoon by a pronounced modulation of the monsoonal
westerlies. These 30-50 day fluctuations of the monsoonal westerlies are
coherent with rainfall and OLR across northern Australia. The OLR fluctuation
originates in the Indian Ocean and systematically propagates eastward at 5
m s-1, consistent with previous studies of the intraseasonal
oscillation.

The detailed evolution of the intraseasonal oscillation of the monsoon is
studied via composites of upper air data in and about the Australian tropics.
During the summer periods 1957-87, 91 events were identified at Darwin,
Australia. The composite oscillation at Darwin has a very deep baroclinic
structure with westerlies extending up to 300 mb. The westerly phase lasts
about ten days and lags a similar duration rainfall event by about four days.
During the westerly phase, the upper troposphere is warm and the extreme lower
troposphere is cool. This structure is consistent with midtropospheric latent
heating and lower tropospheric cooling due to evaporation of falling rain. The
magnitude of the composite oscillation at Darwin is about 5 m s-1 in
zonal wind, 1 m s-1 in meridional wind, 0.5°K in temperature,
5 mm rainfall per day, and 10% in relative humidity. The oscillation at Darwin
is readily traced as far west as Cocos Island and as far east as Pago Pago.

Above northern Australia, enhanced synoptic scale variability develops during
the wet-westerly phase of the oscillation. Analysis of a single station record
precludes documentation of the structure of these synoptic fluctuations. In the
Northern Hemisphere midlatitudes, a wave train in 500 mb heights appears to
emanate from the longitude of the Australian tropics during the wet-westerly i
phase. The magnitude of this wave train is only about 50 m while the wave train
undergoes a systematic evolution as the tropical convective anomaly moves west
to east, no sense of dispersion from a localized low-latitude heat source is
evident.